1. Field of the Invention
The present invention concerns an apparatus for coating one or both sides of a moving fiber web, in particular a paper web or a carton (i.e., cardboard) web, with a liquid or pastous coating material, the apparatus including a coating station for applying a coating onto the web and a non-contact turning apparatus following, viewed in the web""s moving direction, the coating station.
2. Description of the Related Art
From WO 98/32921 an apparatus for coating a paper web on one side which includes a coating station and a non-contact turning station is known. After the coating station, in which a coating material is applied onto the moving paper web with the moving paper web being supported by a backing roll, the paper web moves on to a turning roll. From there it travels to a non-contact turning and drying apparatus and moves therethrough. After leaving the non-contact turning and drying apparatus, it is then led to calender-like cylinders which contact and thereby dry the paper web completely. The paper web is then moved on to further processing. In WO 98/32921, for example, an arrangement of four such coating apparatuses in successive sequence is shown, wherein, onto each side of the paper web, a coating material is applied twice. One disadvantage of this prior art coating apparatus is, on the one hand, that the coating is negatively influenced by turning the humid paper web before it has at least partially dried. This disadvantage tends to be even more pronounced since, due to the construction space available, turning rolls with small roll diameters are usually used which leads to a sharp bending of the paper web at the turning point.
In addition, the use of such a turning roll makes it impossible to convert a coating apparatus to an apparatus with which double-coating can be realized since, in this case, the turning roll would be in direct contact with the humid coating which would intolerably influence the coating""s quality.
The combination of a non-contact turning apparatus and a non-contact air dryer shown in WO 98/32921, in connection with the non-contact turning and drying apparatus, is, for example, already known from DE 295 11 089 U1. From EP 0 770 731 A1, it is further known to locate a turning apparatus and a drying apparatus under one common housing in order to prevent humid exhaust gas and/or humid exhaust air from escaping into the machine hall.
What is needed in the art therefore is a coating apparatus which at least reduces, if not even eliminates, the risk of quality losses of the coated web.
This reduction is achieved according to the invention by a coating apparatus in which the web leaves the coating station and enters the non-contact turning apparatus while changing direction in one plane only. This means that the web moves in free motion without applying an external force from the point of leaving the coating station to the point of entering the non-contact turning apparatus, thereby guaranteeing that the coating layer applied onto the web in the coating station can be fixed in the turning apparatus while maintaining the quality obtained in the coating station.
Here, xe2x80x9cleaving the coating stationxe2x80x9d refers to the point at which the web separates from the web guiding element, serving the purpose of directly applying a coating onto the web. Such a guiding element can be, for example, an applicator roll, a support roll or a band endlessly running around a shoe or similar devices. Decisive is that the web""s coating condition is changed at the guiding element concerned and, with the exception of some desired drying effects, remains unchanged from the point of separation from it.
When referring to the web entering the non-contact turning apparatus, the point at which the turning apparatus influences the web noticeably, i.e. the point at which gas jets from the turning apparatus or infra-red rays hit the web in such a way that the web""s temperature is changed, is referred to.
In the present invention the length of the web""s free motion, i.e. the length of the path of the web between two guiding elements which are in contact with the web, is extended compared with the coating apparatuses known from prior art. This is even more extraordinary since many experts are of the opinion that, with an increase in the length of the free motion, the instability of the web""s path also increases and that therefore the expert should be urged to reduce the length of free motions. Surprisingly, the advantageous effects of the present invention can be achieved without taking additional measures or installing further components for increasing the stability of the web""s path when moving through the coating apparatus.
In addition to the fact that quality impairing effects on the web during turning are avoided, a further advantage can be achieved by the coating apparatus of the present invention: in contrast to the coating apparatus known from prior art, it is now possible to apply at a coating station a coating material onto both sides of the web because of the planar and externally unaffected path of the web after leaving the coating station. Thus, the space, especially the longitudinal dimension required by the coating apparatuses in which a web is to be double-coated once or several times, can be considerably reduced. This means that valuable construction space can be saved.
The coated web""s quality can be further improved by having the web leave the coating station in an upward direction, preferably in an essentially vertical direction. It is often the case that when the web separates, for example, from the applicator roll or a similar coating element used for indirectly applying a coating material onto the web, an effect called xe2x80x9cmistingxe2x80x9d occurs, i.e., a vapour of small coating beads or droplets is formed in the wedge between the web and the surface of the coating element. If the web is, however, leaving the coating station in an upward direction, preferably in an essentially vertical direction, the probability that the small beads fall on the coated web decreases with the increasing pitch angle. The beads are rather more likely to fall back on the coating element instead.
It is, however, also conceivable that the web could leave the coating station in a downward direction. This can be especially helpful in cases where already existing coating apparatuses and/or devices with non-contact turning apparatuses are to be converted in such a way that the web is traveling in one plane only without changing direction between leaving the coating station and entering the non-contact turning apparatus.
As has already been mentioned, the free motion of the web after the coating station serves the purpose of extracting humidity from the web. If the web is leaving the coating station in an upward or downward direction, there is a sufficient path for the web to dry without having to increase the length of the coating apparatus and, with it, its need of space since, in this case, the turning apparatus can be located either essentially above or underneath the coating station.
The non-contact turning apparatus can turn the web by approximately 90xc2x0. This is advantageous if web processing units of great length, e.g. a calender, follow the turning apparatus. The web can, for example, leave the coating station in a vertically upward direction, be turned by the turning apparatus by 90 xc2x0 to a machine running direction and then be led back to the height level of the coating station by a vertically arranged calender. In doing so, the available construction space can be used optimally by arranging the web processing units in a compact way. The machine running direction is the direction the web travels, starting with unwinding the web supply to be coated to reeling in the coated web.
In case no further web processing units are to follow the non-contact turning apparatuses, a sufficient drying length can also be obtained by having the web turned by approximately 180xc2x0 by non-contact turning apparatuses. Use of such a set-up means that about twice the distance between coating station and non-contact turning apparatus is available as drying length.
If only a limited construction height and/or depth (depending on whether the web is leaving the coating station in an upward or downward direction) is available, the required height of the coating station can be reduced if the first turn carried out in the turning apparatus is directed to the machine running direction. If the first turn is in a direction opposite to the machine running direction, the web would inevitably have to be turned again to travel in the machine running direction, a step which could only be carried out above or underneath the first turn of the web.
The above-described turn of the web can be carried out in the turning apparatus in an easy way if the turning apparatus includes a turning unit which is located at the concave side of an turning section of the web. The turning unit can be, for example, a so-called xe2x80x9cairturnxe2x80x9d.
A particularly stable travel of the web which, for example, is desired in cases where the web travels in free motion, can be achieved if the turning apparatus includes a stabilizing unit which is located at the opposite side of the turning unit, on the convex side of the turning section. Such a stabilizing unit can be a non-contact drying unit in the form of a forced convection hood, for example, which will provide the same or differential drying from both sides of the web simultaneously, when considered together with the drying effect of the air turn unit.
A decisive factor of the profitability of, for example, paper or carton processing machines is the speed at which the web to be processed is moving. This moving speed can be increased without impairing the coated web""s quality if at least one non-contact drying unit is, viewed in the web""s moving direction, located before and/or after the turning unit. In doing so, the drying apparatusxe2x80x3 drying performance in relation to the web""s path can be increased, thus enabling a reduction of the time the web is in the drying apparatus.
In case the web is double-coated, the length of free motion can be reduced or the moving speed can be increased if at least two non-contact drying units are located opposite each other on different sides of the web. The at least one drying unit can be an air dryer, an infra-red dryer and/or an infra-red pre-heater. It is also possible to use both infra-red dryers and air dryers in the turning apparatus. It can be advantageous to first pre-dry the humid web by means of infra-red rays, thus making the web insensitive to the subsequently impacting air jets or air flows.
Thereafter, the web can be dried by air drying, which due to the turbulences of the air jets near the web""s surface, is very homogeneous. It can also be advantageous to first pre-heat the humid web with infra-red rays prior to drying with the impinging air jets, in order to increase the rate of drying under the jets and thereby promote improved final coat quality.
In connection with drying the web to a certain degree and at a certain point of the web""s path, it can be desirable to extend the free motion, depending on the degree of the coated web""s humidity in order to obtain more time for the web to dry while traveling at constant speed. This can be achieved in a space-saving manner by providing, viewed in the web""s moving direction, at least one further non-contact turning unit after a first non-contact turning unit. In addition, at least one of the non-contact turning units can also have a drying function in addition to turning the web. For example, such drying can be achieved by heating the web with a gas which is warmer than the impingement air""s ambient temperature and/or increasing the impingement jet velocity of the air turn unit, thereby increasing the Reynolds number of the impingement flow at the product surface. With this turning apparatus, the forced convective drying effect is increased.
As the functions of drying and turning can be combined in one apparatus, it is also conceivable to construct a combined drying and turning apparatus by constructionally combining a drying unit and a turning unit in one element.
During the drying process of the coated web, the coating, which at first had been applied in a humid or pastous form, emits humidity and/or solvents into the atmosphere which, if mixed with the ambient air in the machine hall, may have a harmful effect on the health and well-being of the personnel working in the machine hall as well as on the building itself. These harmful effects can be avoided if the non-contact turning unit, the at least one further turning unit and/or the at least one non-contact drying unit are located in one common housing which, if desired, can consist of several housing parts. The common housing can, for example, have a suction hood sucking off the humid or/and solvent-containing exhaust gas directly from the area around the web such that it cannot get into the ambient atmosphere of the machine hall.
The coating station used for applying the coating can exhibit at least one coating unit for directly applying the coating material onto the web and/or at least one coating unit for indirectly applying the coating material onto the web. This guarantees a desired coating result taking into consideration the available construction space. If only little space for the coating station is available and if coating on both sides is desired, one side of the web can, for example, be coated indirectly by an applicator roll and the other side can directly be coated by a coating unit, wherein the applicator roll serves as a support roll for the coating unit. In addition, the use of any kind of coating device desired is conceivable, such as coating devices known by experts as JetFlow F, SDTA (Short Dwell Time Applicator), LDTA (Long Dwell Time Applicator), Curtain Coater, etc.
If the web is to be coated in several layers, two or more coating units according to the invention can be arranged in succession in one coating apparatus, wherein each individual coating unit can apply a coating layer on one or both sides of the moving web.
The gas-nozzle arrangements used in turning and drying apparatuses play an important role for the operatability of such coating apparatuses. It is, for example, important that there is a stable air cushion at the turning point of non-contact turning apparatuses, such as airturns. The requirements to be met by such an air cushion and its stability increase with an increasing length of the web""s free motion. If an inhomogenity, for example, by a varying degree of the web""s humidity in latitudinal direction is to be avoided, a uniform gas distribution over the length of a gas-nozzle arrangement or a gas distributor is also very important. Also of importance in this respect is the distribution, velocity and direction of the air exiting from the air cushion region.
From the prior art WO 98/5698, certain gas distributors used for drying paper webs are known, each of which is essentially made of an elongated hollow body extending in its longitudinal direction perpendicularly to the web""s moving direction. This hollow body includes a gas intake and a gas exhaust, wherein the gas exhaust is located at the wall section of the hollow body (gas exhaust section), which is essentially parallel to the web""s surface and faces the moving web. The gas distributor of the prior art is characterized in that the introduction of gas takes place at the side wall. The height of the gas-flow relevant cross-section inside the gas distributor decreases linearly from the gas intake side to the side opposite this gas intake side. A disadvantage of this gas distributor is that a homogeneous gas flow over the entire gas distributor""s length is not always guaranteed. This circumstance was tried to be compensated by linearly decreasing the gas-flow relevant cross-section, which, however, again due to the asymmetric gas intake from one side wall, leads to an unnecessarily large construction volume of the gas distributor.
In order to save construction space as well as to guarantee a uniform gas discharge over the entire length of the gas distributor, it is suggested in the present invention to use an above-described gas distributor of the generic type of the hollow body in non-contact turning or/and drying apparatuses for moving webs.
The gas distributor does not necessarily have to extend in its longitudinal direction transversely to the web""s moving direction. It can also be arranged parallel to the web""s moving direction in its longitudinal direction in order to affect certain zones, such as the web""s margin and, for example, to dry them. In addition, it can have any intermediate position desired. This gas distributor has, taken without the coating apparatus described so far, a technical novelty with a value of its own.
Locating the gas intake at the longitudinal center of the elongated hollow body means that the distance from the gas intake point to the most remote gas outlet point is halved, making it more likely that the gas exhaust flow is homogeneous over the entire distributor""s length.
A smaller width of the gas distributor, which may be desirable if several gas distributors are to be arranged next to each other in a small space, can be achieved by locating the gas intake at the hollow body""s wall section opposite the gas exhaust section.
The elongated hollow body can have various forms. Constructing the hollow body as a hollow parallelepiped body can be a particularly simple and cost-efficient version. However, if the vertical clearance of the gas flow cross-section inside the hollow body starting from a longitudinal center section including the gas intake to the two longitudinal ends of the hollow body decreases and preferably does so continuously, the homogenity of the gas exhaust flow can be increased even further. Advantageously, the vertical clearance of the gas flow cross-section of the hollow body at a certain longitudinal position changes proportional to the amount of gas which is to be emitted at that longitudinal position of the hollow body. As a further measure to increase the homogenity of the discharged gas flow, the hollow body can be symmetric with respect to its longitudinal center plane.
From WO 98/56985 an apparatus for drying a paper web with gas distributors arranged parallel to each other and perpendicular to the web""s moving direction under one common gas suction hood is known. However, in the disclosed drying apparatus therein, the gas distributors thereof are supplied with gas from a side wall, leading to an unnecessarily high construction of the drying apparatus.
As already mentioned, the gas distributor of the present invention is particularly suitable for drying or/and turning a moving web within drying or turning apparatuses, in particular in the above-described coating apparatuses. According to the invention, in the turning units at least one gas distributor extends transversely to the web""s moving direction. In contrast to the prior art, the above-described drying and/or turning unit of the present invention needs less construction height. A further advantage is that, in case the web is turned, as large of a latitudinal area of the web as possible is supported by an air cushion. Also, if a gas distributor used in a drying unit is arranged in such a way, as large of a latitudinal area of the web as possible can be affected by drying gas.
As already mentioned, the humid or even solvent-containing exhaust gas is to be sucked off the drying and/or turning area in order to avoid possible impairing effects on the personnel within and the structure of the machine hall near the apparatus. These potentially impairing effects can, for example, be minimized if the turning apparatus, in particular a drying or/and turning unit contained in it, includes a gas suction hood which surrounds the at least one gas distributor, the gas suction hood being open in direction towards the web and being connected to at least one gas exhaust line. The at least one gas distributor is then supplied by a gas supply line. An especially safe turning and a homogeneous drying of the web can be achieved if the at least one gas distributor essentially extends over the entire width of the moving web.
If the web""s moving speed is to be increased without impairing drying performance and without losing any stability at the turning point, a plurality of gas distributors may be arranged under the gas suction hood parallel to each other and spaced apart in the web""s moving direction. Arranging the plurality of gas distributors in such a spaced-apart manner guarantees that the unit is provided with slots or openings through which humid and/or solvent-containing exhaust gas can be sucked off by the gas exhaust line.
In order to keep the effort for processing the gas suction hood as low as possible if several gas distributors are used under one gas suction hood, a central gas supply line can be introduced into the interior of the gas suction hood, wherein advantageously distribution lines inside the gas suction hood would lead from the at least one gas supply line to the individual gas distributors. A single gas supply line is not able to adequately supply an arbitrary number of gas distributors. Depending on the number of distributors, it can therefore be practical to divide the plurality of gas distributors into groups, wherein advantageously one gas supply line and one gas exhaust line are allocated to each group, each such supply line supplying the gas distributors of the corresponding groups with gas and each such gas exhaust line sucking humid and/or solvent-containing exhaust gas of the web""s surface.
In case there is only one gas suction point through which gas from a large area, compared to the cross-section of the gas exhaust line, is sucked off, a non-uniform gas exhaust over the area may occur. It may be that sections near the gas exhaust line experience a stronger gas exhaust than sections further away from this opening. Normally, this is undesirable and can be avoided by providing the gas suction hood with a flow equalizing device for the gas flowing from the moving web into the gas suction hood. The flow equalizing device can, for example, exhibit openings which, in part, have differently sized cross-sections in order to provide sectionwise-adapted flow resistances. A flow equalizing effect can, however, already be achieved if the flow equalizing device is able to provide a constant flow resistance over their entire surface. These flow equalizing devices can be achieved in a material-saving manner by arranging them between the gas distributors parallel to the web""s surface. Each flow equalizing device can thus be made of various structures having minimized material requirements, such as a perforated plate, flow-permeable bonded fiber fabrics, honeycomb structures, grates or the like.
A further possible reason for an inhomogenity in the gas suction flow can be that the gas exhaust lines used for the individual groups perform differently. This effect can be avoided or at least reduced if flow throttle devices are arranged in the gas suction space between the gas distributor groups in essentially a vertical direction and parallel to the gas distributors. These flow throttle devices extend at least over a part of, cross-section of the gas suction hood and advantageously over the entirety thereof. These flow throttle devices can be conceived as being flow-permeable but having flow-resistive partitions between the individual gas distributor groups. These flow throttle devices can also be made of the above-mentioned structures such as perforated plates, honeycomb structures, grates, flow-permeable bonded fiber fabrics, or the like.
Drying a double-coated moving web is a special technical challenge since from nearly the same material volume twice as much humidity has to be removed per time unit. This challenge can, for example, be solved by arranging at least two drying apparatuses or at least one turning and at least one stabilizing unit opposite to each other on different sides of the moving web. This set-up applies not only to drying but also to turning the web, namely in cases where a particular stability of the web is required, such as in case of long free motions of the web. The at least two drying units or the at least one turning and the at least on stabilizing unit can be arranged in such a way that at least some of the gas distributors are located on both sides of the web at essentially the same longitudinal positions, viewed in the web""s moving direction, in pairs opposite to each other. This arrangement has the advantage of being able to dry the web particularly gently since the forces applied on the web by the gas flows compensate each other mutually. A flexing of the web can thus be avoided.
It is, however, also possible to arrange at least some of the gas distributors on one side of the web and at least some of the gas distributors on the other side of the web alternatingly with respect to their longitudinal positions viewed in the web""s moving direction. In other words: a first gas distributor is located on one side of the web; and viewed in the web""s moving direction, spaced apart from the first gas distributor, another gas distributor is located on the other side of the web, etc. With this arrangement the web may, under certain circumstances, experience flexing. However, this arrangement has the advantage that the web""s surface in the drying section is extended by this flexing such that a greater drying performance can be achieved without having to change the construction or running parameters of the drying and/or turning apparatus. It is also possible to arrange some of the gas distributors in one way and some of the other distributors in the other way, i.e. they can be arranged at the same longitudinal positions opposite to each other in some areas and alternatingly in others. This varied arrangement can, for example, be applied at a humid web which had just been coated. This web can initially be guided through an arrangement of alternating gas distributors resulting in the forming of waves on the web, thus increasing its surface. This increased surface increases the drying performance in the drying apparatus and then, with an already pre-dried coating, the web is smoothly moved through the drying apparatus"" gas distributors opposite to each other.
Summarizing, the air turn has been specifically designed to ensure that stable support of the moving web occurs by the generation of a pressure support cushion of dynamic air which comes into equilibrium with the operating web tension at the desired flotation height above the air turn surface. The resulting supporting cushion pressure generated has been made to be substantially independent of the impingement velocity at the nozzle exit to ensure high heat and mass transfer rates are achievable. This situation is brought about by varying the pressure of the gas inside the air turn elements in order to attain the requisite impingement gas velocity at the web surface. The desired web flotation height meanwhile is maintained by regulating the velocity of air exiting the pressure cushion region by ensuring that it flows through a variable gas distributor positioned between the impingement elements located parallel to the web surface. By use of such a device, the distribution of the air exiting from the cushion pressure region is also distributed evenly across the width of the unit while being maintained at the desired cushion pressure level.
Advantageously, nozzle systems as described in EP-B1-0 728 285 may be used, the disclosure of which document hereby is incorporated into the present application by reference. By using such nozzle systems, it is possible to substantially shorten the drying lengths required and, hence, the space taken up in the machine running direction by the overall apparatus.